This invention relates in general to wireless communication systems and equipment, and more specifically to a method and apparatus for controlling a voltage controlled oscillator for use therein.
It is desirable for a voltage controlled oscillator (VCO) used in a portable communication device to meet various conflicting requirements. Such requirements include, for example, low phase noise in order to satisfy sideband noise requirements, and fast start-up of the VCO when leaving a battery-saving mode as well as when switching to a different frequency band, in order to avoid delaying the start of a communication. In prior-art VCOs operating in open loop, guaranteeing startup has implied using a high gain in the VCO amplifier. This high gain in the VCO amplifier in turn has produced a large output swing. Although a large output swing may satisfy the phase noise specifications, it results in wasted power, which is unacceptable for portable communications equipment. A low-gain in the VCO amplifier implies better spectral purity (and fewer harmonics) since the VCO amplifier does not saturate or xe2x80x9cclipxe2x80x9d the output signal. However, with low gain, start-up may not be guaranteed. Thus, a trade-off or compromise has had to be made between the start-up requirement and the phase noise (or output amplitude). Some prior-art VCOs have used a xe2x80x9cmediumxe2x80x9d gain, e.g., a small-signal total loop gain of two or three, as a compromise. Unfortunately, the compromise has not produced very satisfactory results in terms of either start-up speed or phase noise.
Thus, what is needed is a method and apparatus for controlling a VCO that decouples the relationship between guaranteed start-up and phase noise, and achieves reduced phase noise and a fast start-up, independently. The method and apparatus preferably will lend itself to fabrication as a low-power CMOS integrated circuit.